There is one point connected with individual differences, which seems to me extremely perplexing: I refer to those genera which have sometimes been called “protean” or “polymorphic,” in which the species present an inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and several genera of Brachiopod shells. In most polymorphic genera some of the species have fixed and definite characters. Genera which are polymorphic in one country seem to be, with some few exceptions, polymorphic in other countries

Darwin, C. (1859). On the Origin of Species: 46.

1959

[The variety of colour and of shape in the fauna of Arabia might well put anyone skilled in painting to the test, not only in the case of powerful and noble animals but even of the more insignificant